SQU Med J, May 2011, Vol. 11, Iss. 2, pp. 252-258, Epub. 15th May 11
Submitted 27th Jul 10
Revision ReQ. 11th Oct 10, Revision recd. 6th Nov 10
Accepted 1st Dec 10

Department of Microbiology & Immunology, College of Medicine & Health Sciences, Sultan Qaboos University, Muscat, Oman.
*Corresponding Author email: basil@squ.edu.om

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

 :مفتاح الكلمات

aBstract: Objectives: This study aimed to determine the disinfecting potential of some contact lens solutions 
used by some university students in Oman. Methods: This work was carried out from January to June 2010 in the 
Department of Microbiology & Immunology, College of Medicine and Health Sciences, Sultan Qaboos University, 
Oman. Fifty disinfecting solutions, in which contact lenses were disinfected according to the manufacturers’ 
instructions, were collected from the students and plated on various microbiological culture media. Bacterial 
isolates were identified by API-20E, API-20NE and Phoenix automated systems while fungi were identified by 
their cultural characteristics and biochemistry. Results: From 98 isolates, Pseudomonas aeruginosa was 23.5%; 
Penicillium, 13%; Candida species, 9.2%; coagulase negative staphylococci, 9.2%; Serratia marcescens, 6.1%; 
Bacillus, 5.1%; Aspergillus flavus, 5.1%; Serratia liquefaciens, Pseudomonas fluorescens, Enterobacter cloacae and 
Aspergillus niger, 4.1% each;  Chryseomonas luteola and Chryseomonas indologenes, 3.1% each; Stenotrophomonas 
maltophilia, Serratia odorifera, 2.0% each; Enterobacter aerogenes and Klebsiella pneumoniae, 1% each. Most 
isolates (65%) came from polyhexanide containing solutions. Conclusion: Contact lens disinfecting solutions with 
the same formulations, but manufactured by different companies, possessed different disinfecting potentials.

Keywords: Lenses; Disinfecting; Bacteria; Contamination; Fungi; Oman

The Disinfecting Potential of Contact 
Lens Soutions used by Sultan Qaboos 

University Students
*B. C. Nzeako and Sara H. Al-Sumri

CLINICAL & BASIC RESEARCH

Advances in Knowledge
1. This is the first time this type of study has been done at Sultan Qaboos University.
2. It was observed that different disinfecting solutions for contact lenses are used by students at Sultan Qaboos University.
3. Contact lens disinfecting solutions with the same formulation, but manufactured by different companies, possessed different disinfecting 

activity.
4. There is great need to revisit the US Food and Drug Administration guidelines on the use of multipurpose disinfecting solutions for 

contact lenses and storage cases.

Application to patient care
1. Manufacturers’ guidelines for the decontamination of contact lenses and  storage cases should be rigorously followed by the wearers.



B. C. Nzeako and Sara H. Al-Sumri

Clinical and Basic Research | 253

The soft contact lens industry has expanded rapidly over the past four decades because of the demand for a 
convenient alternative to wearing spectacles for 
various purposes. Contact lenses wear can be for 
correction of eye defects (myopia, hypermetropia, 
astigmatism and presbyopia), may be cosmetic 
(decorative) or therapeutic (for the treatment and 
management of bullous keratopathy, dry eyes, 
corneal ulcers and keratitis).1  It is estimated that 
125 million people worldwide use contact lenses 
of which 28–38 million are from USA and 13 
million from  Japan.1 Soft hydrogel contact lenses 
are categorised according to their structure, water 
content, oxygen permeability and mode of wearing 
(daily wear, removed each night), extended wear 
(worn for 6 nights) and continuous wear (worn for 
30 consecutive nights).1-2 Their ability to aid vision, 
give comfort to the wearer and prevent microbial 
keratitis is highly advocated.2-3 Silicone hydrogel 
contact lenses were introduced in 1999 for this 
purpose. Unfortunately, they are not better than 
other soft contact lenses for controlling microbial 
keratitis. Corneal infections continue to be the most 
serious complication of wearing contact lenses.4-10      

 However, many people who wear contact 
lenses are unaware of the likelihood of developing 
eye infections for some bacteria colonise and form 
biofilms inside lens storage cases.3 In this state, 
they become resistant to disinfecting solutions.11-12

A study on 252 soft contact lenses, lens 
storage cases and disinfecting liquids found that 
84.1% of the contact lenses, 80.9% of the lens 
storage cases and 63.1% of the disinfecting liquids 
were contaminated by Staphylococcus aureus, 
Staphylococcus epidermidis, Viridans streptococci, 
Klebsiella pneumoniae, Pseudomonas aeruginosa, 
Pseudomonas fluorescens, Citrobacter amalonaticus 
and Stenotrophomonas maltophilia.13 Another 
study found 9% of lenses, 34% of lens storage cases 
and 11% of lens solutions contained Serratia spp, 
S. aureus, coagulase negative staphylococci and P. 
aeruginosa.14 The contamination was traceable to 
users’ dirty hands, or the tap water used to rinse 
the lens storage cases, and/or air contamination 

during drying of the cases.
Flynn et al15 found contact lenses to harbour 

mostly Gram negative and coagulase negative 
staphylococci. Their mode of adhesion to lenses 
was through deposits of proteins, mucins, lipids 
and inorganic compounds produced by the eye.16-19

 However, the results of studies on the efficacy 
of some contact lens solutions as effective 
disinfectants are conflicting.20-21 The Federal Drug 
Administration (FDA) recommended the mode of 
assessing the efficacy of multipurpose contact lens 
disinfecting solutions using ‘stand alone’ (ISO/
CD 14729) testing procedures. In this procedure, 
all multipurpose solutions are required to ensure 
a 3-log reduction in numbers on three bacterial 
strains (S. aureus (ATCC 6538), Serratia marcescens 
(ATCC 13880) and P. aeruginosa (ATCC 9027), 
1-log reduction on Candida albicans (ATCC 
10231) and Fusarium solani (ATCC 36031).22 
Although the recommendation ensured good 
disinfection of lenses by achieving 3-log reduction 
in cell numbers,21,23 some researchers found many 
laboratory isolates viable in the solution20,24,25 while 
others observed that many microbes associated 
with microbial keratitis were not represented in the 
approved panels of microbes used for the test.26-27       

Disinfecting solutions containing polyhexanide 
were found to kill Escherichia coli, S. epidermidis, 
P. aeruginosa, and S. marcescens while solutions 
containing biguanides killed E. coli and S. 
epidermidis, but not C. albicans.28 The reduced 
efficiency of some disinfecting solutions may be 
attributable to their formulations and mode of 
use.29 

The aim of this study was to assess the 
disinfecting potential of some contact lens 
solutions used by some students at Sultan Qaboos 
University. It is envisaged that the results of 
this investigation can help establish the type of 
microbes in the solutions so that wearers can 
appropriately be advised.

2. Wearers of contact lenses should be aware of the risk of developing microbial keratitis and corneal ulcers.
3. Good personal hygiene during decontamination of lenses and storage cases and during removal or placement of lenses on the eyes is 

essential.



The Disinfecting Potential of Contact Lens Soutions used by Sultan Qaboos University Students

254 | SQU Medical Journal, May 2011, Volume 11, Issue 2

Methods
This study was carried out from January to June 2010 
in the Department of Microbiology & Immunology, 
College of Medicine and Health Sciences, Sultan 
Qaboos University, Oman. Fifty disinfecting 
solutions, in which contact lenses were disinfected 
according to the manufacturers’ instructions, were 
collected from the students and plated on various 
microbiological culture media. No patient or 
patient’s sample was utilised.

Each contact lens user was given three sterile 
bijou bottles, one for the disinfecting solution for 
the contact lens of the right eye, the second for the 
left eye, while the third served as a control (same 
as solution for disinfecting right or left eye lenses). 
This was done to check the degree of sterility of 
the solutions before immersion of the lenses. The 
lenses were immersed in the solutions at night 
and brought to the laboratory in the morning. 
Twelve brands of disinfecting solutions marketed 
by different manufacturers were investigated. 
The solutions were coded 1-12 to mask the 
manufacturers’ names and to avoid brand name 

promotion.

i n o c u l at i o n o f m e d i a
Fifty microlitres of each solution were streaked on 
blood agar (BA, Oxoid, UK), cystine electrolyte 
deficient (CLED, H-Media Laboratories, India), 
and Sabouraud (SAB, Biotec, UK). All the plates 
were incubated at 370C for 48 hrs except Sabouraud 
plates which were incubated at room temperature 
for one week. Bacterial growths on the plates were 
identified using API 20-E, 20-NE (Biomerieux, 
France) and Phoenix automated system (Bacton 
Dickinson, Maryland, USA) while fungal growths 
were identified by their growth characteristics, the 
colour of aerial spores and structural differences 
using lactophenol cotton blue.

The FDA mode of testing disinfecting solutions 
for contact lenses (ISO/CD 14729) was not  
followed because of non-availability of the 
test organisms. However, any solution that 
allows growth of any microbe was regarded as 
contaminated and the contaminating organism 
was identified.  

Table 1: Active agents in some contact lens disinfecting solutions and organisms isolated from them

Solutions (code)  Active agent Samples (N) Growth (N) Isolate

Code 1 Polyhexanide (0.0001%) 15 7 P. aeruginosa
S. liquefaciens
 C. indologenes

Code 2 Polyhexanide (0.0001%) 14 4 P. aeruginosa 
S. marcescens 
Penicillium species

Code 3 Polyhexamethylene biguanide (0.0001%) 1 1 P. aeruginosa

Code 4 Tetronic sulfactant 2 2 P. aeruginosa 
A. flavus 
A. niger

Code 5 Boric acid 2 1 A. niger

Code 6 Polydronium chloride (0.001%) 2 2 P. aeruginosa 

Code 7 Hydroxyethyl methacrylate 1 0 No growth

 Code 8 Polyaminopropyl
Biguanide

6 0 No growth

Code 9 Polyhexanide (0.0001%) 4 2 Bacillus, Penicillium 
species

Code 10 Polyaminopropyl biguanide (0.0001%) 1 1 Penicillium

Code 11 Polyhexamethylene biguanide 1 0 No growth

Code 12 Polyhexamethylene biguanide 1 0 No growth

Totals 50 20 (40%)



B. C. Nzeako and Sara H. Al-Sumri

Clinical and Basic Research | 255

Results
Although the sample sizes of the solutions were 
small and their manufacturers different, the same 
formulations marketed by different manufacturers 
gave different results [Table 1]. Forty percent (40%) 
of the solutions showed growth of various types of 
microbes. Solutions containing polyhexanide had 
65% growth and were used by 66% of the students. 
This was followed by polyaminopropyl biguanide 
with 5% growth and used by 14% of the students. 
All the microbes contaminating control solutions  
were present in the solutions used for the right 
or left contact lenses, but not all the isolates 
contaminating the right or left  contact lenses 
solutions were present in the control solutions 
[Table 2]. However, where the lens solutions and 
their aliquots (controls) were sterile, no organism 
was found. P. aeruginosa (23.5%) and Penicillium 
spp. (13.3%) were the most common isolates while 
Klebsiella pneumoniae and Enterobacter aerogenes 
were the least, 1% each [Table 3].   

Discussion
Soft hydrogel contact lenses are used for various 

purposes (corrective, cosmetic or therapeutic) and 
are either for daily, extended or continuous wear. 
Users are advised to clean their contact lens cases 
and change disinfecting solutions daily except if they 
are silicone hydrogels for extended or continuous 
wear. Contact lenses offer some advantages over 
spectacles in terms of convenience and better visual 
acuity. However, the wearing of contact lenses may 
lead to serious complications including microbial 
keratitis and corneal ulcers which may lead to 
blindness.7-9,30-31   

In this experiment, polyhexanide containing 
solutions, although greater in number, were the 
most contaminated. In contrast, polyaminopropyl 
and polyhexamethylene biguanides inhibited the 
growth of some microbes and allowed growth of 
others. Though their sample sizes were few, they 
possessed more antimicrobial properties than 
polyhexanides. This finding agrees with Santos et 
al.3 and Hume et al.,23 but disagrees with Cano-
Parro et al.28 who found polyhexanides better than 
biguanides. 

In this study, P. aeruginosa had a prevalence 
of 23.5%. The factors contributing to its survival 
in some lens disinfecting solutions were traceable 

Table 2: Microbes isolated from various solutions

Solution used for right eye lenses Solution used for left eye lenses Control solutions 

Coagulase negative staphylococci Coagulase negative staphylococci P. aeruginosa
Candida spp

E. aerogenes
P. aeruginosa,
C. luteola

E. cloacae
P. aeruginosa
C. luteola

C. indolegenes
S. marcescens
Penicillium

C indolegenes, 
Candida spp

K. pneumoniae 
S. marcescens

A. flavus
S. liquefaciens

A. flavus Candida spp A. niger

S. hominis
Bacillus
S. liquefaciens
Penicillium

E. cloacae
A. flavus
S. liquefaciens
Candida spp

Bacillus spp

S. marcescens
C. indolegenes
S. marcescens

S. odorifera
S. marcescens
C. indolegenes

A. niger 
A. flavus

A. niger
Bacillus
Penicillium spp

P. fluorescens
P. fluorescens
S. odorifera
E. cloacae

P. fluorescens
S. maltophilia



The Disinfecting Potential of Contact Lens Soutions used by Sultan Qaboos University Students

256 | SQU Medical Journal, May 2011, Volume 11, Issue 2

to its adaptability to adverse environmental 
conditions and capability to attach easily to corneas 
and contact lenses (rigid, hydrogel, high and low 
water content contact lenses).16,32-33

In contrast, Enterobacter, Serratia and Klebsiella 
species which are usually of faecal origin can be 
transferred to the disinfectants by the wearers 
during the process of immersion or removal of the 
lenses from the solutions. In addition, some of the 
organisms like Serratia and Pseudomonas species 
are resistant to some disinfecting solutions.20 Fungi 
like Candida, Penicillium and Aspergillus species 
are adaptive to diverse environments and require 
little moisture and organic substrate for growth. 
They are likely to come from poor or inadequate 
cleaning of contact lens cases since bacteria 
interacting with contact lens storage cases form 
biofilms that make them resistant to disinfecting 
solutions.17 The nutrients for growth are acquired 
from lipids, proteins and glycoproteins present 
in the tears of the eyes.35 However, when they are 
present on the lenses and lens cases, the efficacy 
of the disinfecting solution can be neutralised by 
their presence.11 The isolates contaminating the 
control solutions also contaminated the solutions 
used for disinfecting left and right eye lenses. This 
indicates that the solutions were contaminated 
before the immersion of the lenses. In this study, 
it is observed that some control solutions were 
sterile although microbes were isolated from their 
aliquots used for disinfecting the right or left 
lenses [Table 2]. Such contamination is inferred to 
originate from the user (poor hand hygiene), the 
lenses or from the storage cases. Some researchers 
observed that non-compliance with the guidelines 
for caring for contact lenses and lens cases was a 
major issue in the use of contact lenses.32 Their 
observation was supported by the finding that 11% 
of the contamination of solutions was due to poor 
hand hygiene, 13% to inadequate disinfection of 
lenses, and 61% to inappropriate cleaning practices 
of storage cases.31,32 From whatever source and 
by whatever means, the organisms got into the 
solutions and some of the disinfectants could not 
eliminate the organisms. This experiment appears 
to establish the fact that disinfecting solutions 
for lenses are not sterilising solutions, but agents 
meant to reduce the microbial numbers on lenses 
and cases 

Conclusion
Twelve disinfecting solutions for soft hydrogel 
contact lenses were examined for growth of microbes 
after lenses were removed from the eyes and 
immersed overnight in the disinfecting solutions. 
Forty percent (40%) of the solutions grew some 
microbes, with polyhexanide containing solutions 
showing highest growth (65%). The least growth (5% 
and 5% respectively) came from polyhexamethylene 
and polyaminopropyl biguanides. Because of the 
small and unequal number of samples investigated, 
it is statistically difficult to state which is the best 
disinfecting solution. 

 The small number of samples used in this study 
limits the outcome of the investigation. Further 
work using larger samples and looking for parasites 
like Acanthamoeba is necessary. However, it is of 
importance that multipurpose solutions which 
clean, disinfect and rinse contact lenses and their 
cases be used for all contact lenses.17,21,24,   The lenses 
should be stored dry in their cases after disinfection. 
Before removing the lenses from the eyes, the 
user should wash his/her hands thoroughly in 
soapy water. If a multipurpose solution containing 
hydrogen peroxide is used for disinfecting the 
lenses, the lenses should be rinsed several times in 
saline solution to get rid of the hydrogen peroxide 
which is toxic to the eyes.35 It should be borne in 
mind by all contact lens users that the disinfecting 
solutions do not sterilise contact lenses and lens 
cases, but only reduce the microbial load on them. 
The reduction is only possible if the organisms to 
be reduced are susceptible to formulations in the 
disinfecting solutions. Currently, the performance 
of disinfecting solutions for all types of contact 
lenses is being re-visited and various formulations 
are being suggested.20,21

c o n f l i c t o f i n t e r e s t
The authors reported no conflict of interest.

a c k n o w l e d g e m e n t s

The authors are grateful to the Department of 
Microbiology & Immunology for providing the 
media and the reagents used for the study and for 
their cooperation throughout the duration of the 
work.



B. C. Nzeako and Sara H. Al-Sumri

Clinical and Basic Research | 257

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Table 3: Percentage distribution of isolates from all 
solutions

No. Organisms No. of 
isolates

Percentage

1 P. aeruginosa 23 23.5%

2 Penicillium spp. 13 13.3%

3 Candida spp. 9 9.2%

4 Coagulase negative 
staphylococci

9 9.2%

5 S. marcescens 6 6.1%

6 Aspergillus flavus 5 5.1%

7 Bacillus 5 5.1%

8 S. liquefaciens 4 4.1%

9 P. fluorescens 4 4.1%

10 E. cloacae 4 4.1%

11 Aspergillus niger 4 4.1%

12 C. luteola 3 3.1%

13 C. indolegenes 3 3.1%

14 Stenotrophomonas 
maltophilia

2 2.0%

15 S. odorifera 2 2.0%

16 E. aerogenes 1 1.0%

17 K. pneumoniae 1 1.0%

Total 98 100%



The Disinfecting Potential of Contact Lens Soutions used by Sultan Qaboos University Students

258 | SQU Medical Journal, May 2011, Volume 11, Issue 2

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